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HISTORY OF PHYSICAL ASTRONOMY.

has given a description of the astrolabe in the beginning of the fifthbook of the Syntaxis . It was used for the purpose of determining thelatitude of a celestial body, and its longitude relative to another body,with which it was compared, whose absolute longitude was known. Twoequal circles, whose planes were perpendicular to each other, were firmlyfastened together, and were so disposed that the one represented thecelestial ecliptic, and the other the solstitial colure. At the points of thesolstitial circle, corresponding to the poles of the ecliptic, there wereplaced two cylinders, which projected both within and without the circle.To the outer cylinders was adapted a movable circle of latitude, the in-terior of which coincided exactly with the exterior of the solstitial circle.A similarly movable circle was adapted to the inner cylinders, which wasso constructed as to be embraced exactly by the concave surface of thesolstitial circle. The whole machine was made to turn round the twopoints in the solstitial colure corresponding to the poles of the equator,and its position w ! as adjusted by directing the axis of revolution to thepoles of the celestial equator.

When it was required to determine the position of a celestial bodywith this instrument, by means of another body whose position had beenalready ascertained, the exterior circle was made to revolve until the latterbody appeared in its plane, and the interior circle was similarly directedto the body whose position was to be found. The distance between thesetwo circles, measured upon the ecliptic, then gave the difference of thelongitudes of the two bodies, and the distance of the second body fromthe ecliptic, measured upon the interior circle, gave its latitude *.

The astrolabe thus constructed served to determine the position of acelestial body by means of its longitude and latitude. It is manifest that,by placing a circle in the plane of the equator, the same object might beaccomplished with still greater facility by observing the right ascensionand declination of the body Hipparchus must have used the astrolabe inthis form in the early part of his career, since it is by means of their rightascensions and declinations that he originally indicated the places of thestars. He was doubtless induced to abandon this practice upon hisdiscovery of the precession of the equinoxes, since an obvious advantagethen appeared to result from the designation of the place of a star by itslongitude and latitude.

It does not appear that regular observations of the moon were made byany astronomer previous to Hipparchus . The earlier observers confinedthemselves to notices of eclipses and occultations, and it was by an exami-nation of records of the former class of phenomena, extending over a longsuccession of ages, that the Greek mathematicians arrived at those secularperiods relative to the moon’s mean motion, which Ptolemy has alluded toin the Syutaxis. Hipparchus also, as has been already mentioned, de-duced the elements of the lunar orbit from observations of eclipses, andso long as he confined his researches on the moon’s motion to her syzygees,

or longitudes of two stars. Such observations would have been of no Value unless it hadbeen possible, in each case, to ascertain also the absolute place of either of the stars with re-spect to the equinox. This, however, could not be effected without the use of solar tables(at least accordin'? to any method known in ancient times), and such were not available tothe earlier astronomers of the Alexandrian School, since Hipparchus was the first whoestablished the theory of the sun’s apparent motion.

* The place of the star upon the interior circle of latitude was determined by meansof a smaller concentric circle revolving in the same plane, which was furnished with twoprojecting pinnules diametrically opposite to each other.